1. Field of the Invention
The present invention relates to a single point bonding method; and more specifically to a single point bonding method used in particular for a TAB (Tape Automated Bonding) packaging.
2. Description of Related Art
As shown in FIGS. 1A and 1B, a film carrier tape used in the TAB packaging includes an insulative base film 1 formed of for example polyimide. The insulative base film 1 has two arrays of sprocket holes 2 formed at both edges thereof for conveying and positioning, and a plurality of square device holes 3 (only one shown) in which a semiconductor chip 10 is to be located. The base film 1 has on its one surface a number of leads 6 and a number of test pads 7 for electrical selection or sorting, which are formed for example by adhering a metal foil such as a copper foil on the surface of the base film 1 and etching the metal foil into a desired pattern by means of a photolithography. Each of the leads 6 includes an inner lead 6A projecting into the device hole 3 and an outer lead 6B connected to one test pad 7. The leads 6 are plated with a metal such as a gold, tin, or solder,
Directly under the outer leads 6B, the base film 1 has four rectangular outer lead holes 4 each formed in parallel to a corresponding one of four edges of the square device hole 3 as shown in the drawing. A suspender 5 between the outer lead holes 4 and the device hole 3 holds the leads 6 and is coupled to the body of the base film 1 at four corners.
In order to bond the leads 6 of the film carrier tape to the semiconductor chip 10, a bump 8, which is a metal projection, is previously on each of the electrodes of the semiconductor chip 10, and is positioned directly under a corresponding inner lead 6A to be bonded to the bump. Then a bonding tool (not shown) is activated from an upper position so as to bond the inner lead 6A to the bump 8. This connection manner is called an inner lead bonding (ILB) process.
The semiconductor chip 10 has a square main surface, on which a number of bump 8 are provided along each of edges of the square main surface. However, there may be bumpless electrodes having no projection. The inner leads 6A positioned to oppose the corresponding bumps 8 are fixed on the suspender 5 in units of each edge of the square device hole, and the outer leads 6B are fixed to the body of the base film 1. In a process after the inner lead bonding, the test pads 7 and the base film 1 are removed. In fact, the bumps are greatly more than the number of the shown bumps, and in some cases, some of the leads 6 are previously omitted.
Generally, the ILB process is divided into two kinds: a gang bonding for simultaneously bonding all the inner leads 6A to the bumps by a bonding tool; and a single point bonding for bonding the inner leads 6A to the bump 8, one lead at a time. The former gang bonding becomes difficult to maintain the parallelism between the bonding tool and the semiconductor chip 10, if the size of the semiconductor chip 10 becomes large and the number of the leads 6 also increases. In addition, for adjusting the parallelism, a lengthy setting operation is required at each time the size of the semiconductor chip 10 changes. Therefore, the gang bonding is poor in regard to large pin-numbers which is required in a few-of-a-kind production. The latter single point bonding does not require the fine adjustment required in the former gang bonding, since the inner leads are certainly bonded by the bonding tool, one at a time. Therefore, with a size-increasing and pin-number-increasing inclination of the semiconductor chips, inner lead bonding machines adopting the single point bonding are increasing in number.
When the ILB process is performed using the single point bonding, since the inner leads 6A are bonded one by one, the bonding time becomes long for example if the pin number reaches 200 or more. Because of this, the base film 1 thermally expands due to a heat applied to the bonding tool at the time of the bonding, so that as the bonding of the inner leads 6A advances, a positional shift or deviation gradually occurs. As a result, it becomes difficult or impossible to properly push the inner leads 6A by the boning tool. Therefore, a defective bonding occurs due to the positional shift or deviation, and although the bonding is obtained, the bonding strength becomes insufficient.
According to the technique disclosed in Japanese Patent Application Laid-open Publication JP-A-5-074875 to cope with the above mentioned problem, as shown in FIG. 2A, among a number of inner leads 6A (parts omitted in the drawing) in the device hole 3 along each edge of the square principal surface of the semiconductor chip (not shown), firstly, inner leads 61 positioned at a central portion of one edge are bonded, and then, inner leads 62 positioned at a central potion of the edge opposing to the edge of the inner leads 61 are bonded. Thereafter, inner leans 63 are bonded, and then, inner leads 64 are bonded. This patent application describes that, with this procedure, the thermal expansion of the base film can be dispersed, with the result that the lead positional deviation can be relaxed. The disclosure of Japanese Patent Application Laid-open Publication JP-A-5-074875 is incorporated by reference in its entirety into the present application.
Another prior art, "IMC 1990 Proceedings, Tokyo, May 30-Jun. 1, 1990", Pages 202-207, mentions that, as shown in FIG. 2B, of a number of inner leads 6A (parts omitted in the drawing) along each edge of the square device hole, firstly, an inner lead(s) 71 positioned at a corner is bonded, and then, inner leads 72, 73, 74, 75: 76, 77 and 78 each positioned at one corner are bonded in the named order, and with this procedure, the positional deviation can be prevented. The disclosure of "IMC 1990 Proceedings, Tokyo, May 30-Jun. 1, 1990", Pages 202-207 is also incorporated by reference in its entirety into the present application.
However, neither of the conventional bonding procedures shown in FIGS. 2A and 2B can completely solve the generation of the positional deviation, because sufficient attention is not paid to a relation between the bonding time and the thermal expansion and because deformation of the base film is considered only in a lateral direction (along an edge).
For example, assuming that the bonding is sequentially performed, one at a time, from a left-hand first corner to a right-hand second corner, a conduction heat of the heated semiconductor chip and a radiation heat from the bonding tool are transferred through the inner leads to the base film, so that the base film thermally expands. Therefore, if some degree of bonding time elapses, unbonded inner leads gradually cause the positional deviation so that the inner leads eventually fail to be accurately positioned on the electrodes of the semiconductor chip. In this condition, if the bonding tool is abutted onto the inner lead, a defective electrical and mechanical connection occurs.
In particular, in the case that the central inner leads are first bonded as shown in FIG. 2A, when inner leads at one side of the bonded central inner leads are being bonded one by one, inner leads at the other side of the bonded central inner leads cause the positional deviation due to heat transmitted from the bonded central inner leads.
In the conventional example shown in FIG. 2B in which the bonding of the inner leads is started from both corners (first and second corners) in order to prevent a lateral positional deviation, since the base film expands similarly, in most cases the base film deforms upward into a convex form (in a direction of increasing the distance between the semiconductor chip and the inner leads). As a result, the distance from the inner lead to the corresponding pad electrode becomes long, so that the length of the inner leads becomes insufficient, with the result that a defective bonding may occur. In addition, since the base film is lifted to depict a circular arc, the inner leads at both sides of the top of the circular arc are inclined to the pad electrode surface of the semiconductor chip. If the bonding is performed in this condition, the inner leads cannot be certainly closely bonded on the pad electrode, so that the inner leads slide, resulting in a defective bonding.
The height of the top of the circular arc from the level of the corner portion is on the order of 30 .mu.m to 70 .mu.m (in the case that the thickness of the base film is 125 .mu.m). Accordingly, if the bonding is sequentially performed in only one direction, the inner leads eventually fail to be accurately positioned on the electrode.